Energy storage systems, such as power conditioning systems, are used in a variety of applications, such as providing emergency back-up power or providing consistent power from variable energy sources, such as wind, solar, or other variable energy sources. These energy storage systems can include different energy storage devices, such as parallel-connected battery wings or battery strings. In such applications, a plurality of battery wings are commonly connected in parallel, with each battery wing including a plurality of parallel-connected battery strings. In such a system, a controller, such as a plant controller, typically constrains operation of the battery wings or battery strings based upon the allowable system current rating provided by the plant controller. The allowable system current can be dynamically determined based on the available current capacity for individual battery strings. For example, if an individual battery string is approaching its current rating, the plant controller can reduce the allowable system current for the system to prevent the individual battery string from exceeding its current rating.
Due to the nature of a parallel-connected energy storage system, individual energy storage devices can contribute unequal currents to total current of the energy storage system. For example, as individual energy storage devices degrade over time, or as the operational characteristics of an energy storage device are varied, such as a state-of-charge or temperature, the resistance of an energy storage device can differ from the resistance of other energy storage devices connected to the system. In such a case, an individual energy storage device operating at the same voltage as all other energy storage devices connected to a common bus will have a varied current based on the resistance of the energy storage device. Thus, energy storage devices with lower resistances will contribute higher currents to the total current of the energy storage system, which can cause an individual energy storage device to approach its current rating before the other connected energy storage devices approach their respective current rating. To account for this current discrepancy, a typical solution for preventing any individual energy storage device from operating at a current greater than its current rating is for a controller, such as a plant controller, to derate, or reduce, the allowable system current for the energy storage system. For example, if an individual string is operating at or close to its current rating, the plant controller can reduce the allowable system current of the system to a level that will prevent the individual string from exceeding the current rating. However, derating the allowable system current for the energy storage system can reduce the total power that can be delivered by the energy storage system. Further, when the allowable system current for the system is limited by a single energy storage device, the energy storage system may be unable to meet the energy demands required of the system.